1. Field of the Invention
The present invention relates to highly flexible combustion equipment and a highly flexible process capable of operating with both oxygen and with a combination of oxygen and air, as fuel combustion oxidizer.
2. Related Art
Thermal processes in which the thermal energy comes from the combustion of a fuel face ever stricter environmental constraints, in particular in terms of CO2 and NOx emissions. Examples of such thermal processes are, for example, steam generation in boilers and melting processes in furnaces.
The two most prevalent techniques used to reduce NOx emissions are low-NOx burners and flue gas recirculation (FGR) burner systems.
Low-NOx burners reduce NOx emissions by accomplishing the combustion process in stages: i.e. by staging the injection of the fuel and/or of the oxidizer. Staging partially delays the combustion process, resulting in lower combustion temperatures, it being known that higher combustion temperatures contribute to NOx generation.
The two most common types of low-NOx burners for natural-gas fired boilers are staged air injection burners and staged fuel injection burners. NOx emission reductions of 40% to 85% have been observed with low NOx burners compared to non-staged burners.
In an FGR system, a portion of the flue gases generated by the combustion is recycled from the stack to the burner windbox. The recycled flue gas reduces NOx emissions by two mechanisms. It firstly reduces nitrogen concentrations in the combustion zone and secondly acts as an inert or ballast to reduce the oxygen concentration and thereby the combustion temperature.
FGR is normally used with specially designed low-NOx burners capable of sustaining a stable flame even with increased inert gas flow.
In the present context, the term “inert” is used to refer to substances or flows which, in the given process, do not as such contribute to heat generation by combustion, i.e. the inert gas is neither a combustion fuel nor a combustion oxidizer under the given combustion process parameters.
Examples of known low-NOx burners are described in inter alia WO00/79182, WO 02/081967, WO 04/094902, WO 2005/059438 and WO 2005/059439.
Examples of known FGR burner systems are described in inter alia WO 2008/007016, WO 2009/090232 and WO 2009/136366.
A specific example of a low-NOx combustion method and burner with FGR is proposed in U.S. Pat. No. 5,411,394 on the basis of an experimental parameter study. In accordance with this known method there is provided a burner having a chamber with an insertion region, said insertion region including a low divergence fuel nozzle arranged on a burner axis and first, second and third concentric nozzles. Each said nozzle is arranged at increasing radii from said axis and is arranged to introduce flow to said chamber from substantially the same axial location. Fuel is injected through said fuel nozzle to form a combustible fuel flow along said axis. A concentric flow formed by first, second and third successively concentric component flows, including oxidant gases, is injected through said first, second and third concentric nozzles. The fuel flow and the concentric flow are stratified to limit mixing of oxidant gases with said fuel flow so as to maintain a high-temperature fuel rich core zone near the insertion region and to induce mixing with oxidant gases in a lower temperature recirculation zone spaced from said insertion region. Stratification is achieved by providing a combination of a radial density gradient from low density, high temperature in said core zone close to the axis to higher density, lower temperature spaced radially from said core and by swirling said concentric flow. The fuel pyrolizes in the high-temperature fuel-rich core zone near said insertion region, where the mixing of oxidant gases with said fuel is limited by the stratifying. The product of said high temperature fuel-rich core zone is combusted in the lower-temperature recirculation zone spaced from said insertion region, where mixing of ambient gases is induced. According to U.S. Pat. No. 5,411,394, the use of small amounts of flue effluent in the primary air and/or tertiary air flow to deplete the oxygen concentration of the air flow and the flame temperature is a particular aspect for reducing NOx emissions.
In certain other cases, in particular in furnaces shielded from the ambient nitrogen-rich air atmosphere, oxygen-fuel combustion (as opposed to air-fuel combustion) is an option for reducing NOx generation due to the lower or even zero nitrogen content in the oxidizer. Also due to the lower nitrogen content of the oxidizer, oxygen-fuel combustion can also be of interest for reducing CO2 emissions. Indeed, due to the lower nitrogen concentration and thus the higher CO2 concentration in the oxygen-fuel combustion flue gases, CO2 capture and sequestration becomes an option.
An example of a low-NOx oxygen-fuel burner is described in EP-A-0754912.
From an environmental point of view, there is a clear need for low-NOx oxygen-fuel combustion equipment and processes.
In spite of this, a major portion of thermal processes still rely on air-fuel combustion for the generation of thermal energy, and oxygen-fuel combustion has been slow to conquer the industry.
This can at least in part be explained as follows.
The established air-fuel combustion plants, with which the industrial operator is familiar, often do not have the proper geometry or the proper equipment to operate with oxygen instead of air as the combustion oxidizer. One reason for this is that the use of oxygen instead of air significantly alters the heat transfer modes, the concentrations of combustion product species and the pressure regimes within the combustion chamber and possible downstream heat transfer areas.
Known oxygen-fuel combustion plants likewise do not usually have the proper geometry or the proper equipment to operate with air instead of oxygen as the combustion oxidizer.
When designing or constructing a plant, the operator therefore has to select the oxidizer with which the combustion plant is to operate.
Hence, in spite of the well-known advantages of oxygen-fuel combustion as regards reduced pollution and increased energy efficiency, many plant operators are reluctant to construct oxygen-fuel combustion plants, in particular for those processes for which the environmental constraints currently do not impose or commercially justify oxygen-fuel operation.